Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and click on General settings.
Step::3 Now in the next step jump over to the Ordering tab.
Step::4 Scroll below and find “Allow notes on Checkout” option. Tick to enable.
Step::5 Click on Save changes.
That's It !Check out the video tutorial on the related topic.Read the blog as well.